Sensing Vector Selection in a Cardiac Stimulus Device with Postural Assessment

ABSTRACT

Methods, implantable medical devices and systems configured to perform analysis of captured signals from implanted electrodes to identify cardiac arrhythmias. In an illustrative embodiment, signals captured from two or more sensing vectors are analyzed, where the signals are captured with a patient in at least first and second body positions. Analysis is performed to identify primary or default sensing vectors and/or templates for event detection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/672,353, filed Feb. 7, 2007, published as US Patent ApplicationPublication Number 2008-0188901 and titled SENSING VECTOR SELECTION IN ACARDIAC STIMULUS DEVICE WITH POSTURAL ASSESSMENT, the disclosure ofwhich is incorporated herein by reference.

This application is related to U.S. patent application Ser. No.11/623,472, filed Jan. 16, 2007 and titled SYSTEMS AND METHODS FORSENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE USING APOLYNOMIAL APPROACH, now U.S. Pat. No. 7,783,340, the disclosure ofwhich is incorporated herein by reference. This application is alsorelated to U.S. patent application Ser. No. 11/441,522, filed May 26,2006, published as US Patent Application Publication Number 2007-0276445and titled SYSTEMS AND METHODS FOR SENSING VECTOR SELECTION IN ANIMPLANTABLE MEDICAL DEVICE, the disclosure of which is incorporatedherein by reference.

FIELD

The present invention relates to the field of implantable medicaldevices. More particularly, the present invention relates to implantabledevices that monitor and/or stimulate the heart.

BACKGROUND

Implantable cardiac monitoring and/or stimulus devices can providevarious benefits to patients who receive them. Such devices are adaptedto monitor cardiac activity of a patient while implanted and, if soequipped, to provide stimulus when necessary to assure adequate cardiacfunction. New and different methods are desired for configuring andperforming cardiac signal assessment in such devices.

SUMMARY

The present invention, in an illustrative embodiment, includes animplantable medical device that includes sensing electrodes andcircuitry that allow the device, when implanted in a patient, to senseelectrical activity emanating from the patient's heart along a pluralityof sensing vectors. In the illustrative embodiment, the implantablemedical device is configured to select a primary or default sensingvector by observing cardiac signal characteristics along one or more ofthe plurality of sensing vectors. In an illustrative embodiment,observation of the cardiac signal characteristics includesinitialization in terms of the body position or posture of the patient.

In another illustrative embodiment, a device as described above isincluded as a part of a system including an external programmer, whereinthe programmer and implanted device are configured to communicate withone another. In this embodiment, the system is configured such that thepatient may be directed to perform certain acts and/or assume selectedpostures/poses/body positions via the programmer, allowing the implanteddevice to observe the effects of changes of posture by the patient oncaptured cardiac signal. The implanted device (or the programmer,depending upon the particular configuration) may then select a primaryor default sensing vector.

Another illustrative embodiment includes a method of selecting a vectorfor use in sensing cardiac events. In the illustrative method, sensingcharacteristics along several vectors may be considered with the patientin various body positions (for example, standing, sitting, and/or lyingdown). Using the captured sensing characteristics, a default or primarysensing vector may be selected. In an illustrative example, the methodincludes directing the patient to assume a set of postures/poses/bodypositions.

In addition to selecting a primary or default sensing vector, in someembodiments, a secondary vector is selected for various uses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate subcutaneous and transvenous cardiac stimulators;

FIG. 2 is a block diagram illustrating steps in an illustrative implantprocedure;

FIGS. 3-5 show illustrative methods of postural assessment in animplanted medical device;

FIGS. 6A-6B are graphs of variable relationships in an illustrativemethod of analyzing sensing vectors during postural assessment;

FIGS. 7A-7C are graphs of variable relationships in illustrative signalanalysis methods; and

FIGS. 8A-8E are graphical representations of display outputs of aprogrammer during an illustrative method of postural assessment.

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention.

The present invention is related to U.S. patent application Ser. No.11/441,522, filed May 26, 2006 and entitled SYSTEMS AND METHODS FORSENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE, published asUS Patent Application Publication Number 2007-0276445, the disclosure ofwhich is incorporated herein by reference. In particular, the '522Application shows illustrative methods of analyzing cardiac signalscaptured along a given sensing vector. The methods shown therein areillustrative of analytical methods and “scoring” that may also beperformed in association of with the present invention.

FIGS. 1A-1B, respectively, show subcutaneous and transvenous implantedcardiac stimulus systems relative to the heart. Referring to FIG. 1A,the patient's heart 10 is shown in relation to an implanted,subcutaneous cardiac stimulus system including a canister 12. A lead 14is secured to the canister 12 and includes sensing electrode A 16, coilelectrode 18, and sensing electrode B 20. A can electrode 22 is shown onthe canister 12. Several vectors for sensing are therefore availableincluding A-can, B-can, and A-B. It should be noted that the use of thecoil electrode 18 as a sensing electrode is also possible. Illustrativesubcutaneous systems are shown in U.S. Pat. Nos. 6,647,292 and6,721,597, and the disclosures of these patents are incorporated hereinby reference. Some embodiments include a unitary system having two ormore electrodes on a housing as set forth in the '292 patent, ratherthan that which is shown in FIG. 1A. A unitary system including anadditional lead may also be used. It should be understood that for anyvector discussed herein, either of the two available polarities for eachvector is possible and may be analyzed and/or selected, if desired.

Referring now to FIG. 1B, a transvenous system is shown relative to apatient's heart 30. The transvenous cardiac stimulus system includes acanister 32 connected to a lead 34. The lead 34 enters the patient'sheart and includes electrodes A 36 and B 38. The illustrative examplealso includes coil electrodes 42, shown both internal and external tothe heart 30. The coil electrodes 42 may be used for sensing or stimulusdelivery. In the illustrative example, electrode A 36 is locatedgenerally in the patient's ventricle, and electrode B 38 is locatedgenerally in the patient's atrium. The lead 34 may be anchored into thepatient's myocardium. Again, a can electrode 40 is shown on the canister32. With the transvenous system, plural sensing vectors may be definedas well.

In both FIGS. 1A and 1B, one or more sensing electrodes may also be usedfor stimulus delivery. Some embodiments of the present invention may beused in combination systems that may include sensing vectors definedbetween two subcutaneous electrodes, a subcutaneous electrode and atransvenous electrode, or two transvenous electrodes. For example, thepresent invention may be embodied in a hybrid system having electrodesfor each of several transvenous, epicardial, and/or subcutaneouslocations.

In the configurations of FIGS. 1A and 1B, there are multiple sensingvectors available. Detection of cardiac function along one of thesesensing vectors allows the implanted cardiac stimulus system todetermine whether treatment is indicated due to the detection andidentification of a malignant condition such as, for example, aventricular tachycardia. An implanting physician may perform vectorselection by determining which of the captured vectors is best, forexample by visual inspection of a graphical representation of capturedsignals. However, this requires an assessment of cardiac function alongseveral vectors and may increase the time needed to performimplantation, and also increases the risk of human error. Further, theselection of a vector may require advanced or specialized training, asselection of a suitable vector among those available is not necessarilyintuitive.

Robust sensing vector selection methods are desirable, as well asdevices adapted to perform such methods. The present invention, inillustrative embodiments, provides such methods and uses variouscriteria for doing so. Some embodiments include implantable devices andprogrammers for implantable devices that are adapted to perform suchmethods.

The systems shown in FIGS. 1A-1B may include operational circuitry andpower sources housed within the respective canisters. The power sourcesmay be, for example, batteries or banks of batteries. The operationalcircuitry may be configured to include such controllers,microcontrollers, logic devices, memory, and the like, as selected,needed, or desired for performing the illustrative methods set forthherein. The operational circuitry may (although not necessarily) furtherinclude a charging sub-circuit and a power storage sub-circuit (forexample, a bank of capacitors) for building up a stored voltage forcardiac stimulus taking the form of cardioversion and/or defibrillation.The operational circuitry may also be adapted to provide a pacingoutput. Each of cardioversion/defibrillation and pacing sub-circuitryand capacities may be incorporated into a single device. The methodsdiscussed below may be embodied in hardware within the operationalcircuitry and/or as instruction sets for operating the operationalcircuitry and/or in the form of machine-readable media (optical,electrical, magnetic, etc.) embodying such instructions and instructionsets.

Each of the devices 12, 32 may further include such components as wouldbe appropriate for communication (such as RF communication or inductivetelemetry) with an external device such as a programmer. To this end,programmers 24 (FIG. 1A) and 42 (FIG. 1B) are also shown. For example,during an implantation procedure, once the implantable device 12, 32 andleads (if included) are placed, the programmer 24, 42 may be used toactivate and/or direct and/or observe diagnostic or operational tests.After implantation, the programmer 24, 42 may be used to non-invasivelydetermine the status and history of the implanted device. The programmer24, 42 and the implanted device 12, 32 may be adapted for wirelesscommunication allowing interrogation of the implanted device in anysuitable manner for an implanted device system. The programmers 24, 42in combination with the implanted devices 12, 32 may also allowannunciation of statistics, errors, history and potential problem(s) tothe user or physician.

FIG. 2 is a block diagram illustrating steps in an illustrative implantprocedure. From a start block 100, the first step is the physicalimplantation itself 102, which may include various surgical preparationsas are known in the art, incision of the patient, and emplacement of asystem, for example, a transvenous or subcutaneous system as shown abovein FIGS. 1A-1B. Methods for physically implanting a transvenous deviceare well known. A subcutaneous device may be implanted, for example, asset forth in copending U.S. patent application Ser. No. 11/006,291,filed Dec. 6, 2004 and titled APPARATUS AND METHOD FOR SUBCUTANEOUSELECTRODE INSERTION, now U.S. Pat. No. 7,655,014; and/or copending U.S.patent application Ser. No. 11/497,203, filed Aug. 1, 2006, published asUS Patent Application Publication Number 2008-0046056 and titledELECTRODE INSERTION TOOLS, LEAD ASSEMBLIES, KITS AND METHODS FORPLACEMENT OF CARDIAC DEVICE ELECTRODES, the disclosures of which areincorporated herein by reference.

With the system in place in the patient, the device is initialized, asindicated at 104. This may include various functions as indicated at106, such as power-up of the device, system check, lead connection,detection and impedance checks. Initialization 104 may also includevector selection steps and template formation steps, allowing an initialset-up of the device while the patient is in the operating room.Illustrative methods of template formation are discussed in copendingU.S. patent application Ser. No. 10/999,853, filed Nov. 29, 2004 andentitled METHOD FOR DEFINING SIGNAL TEMPLATES IN IMPLANTABLE CARDIACDEVICES, now U.S. Pat. No. 7,376,458, the disclosure of which isincorporated herein by reference. Illustrative methods of vectorselection are discussed in copending U.S. patent application Ser. No.11/441,522, filed May 26, 2006, published as US Patent ApplicationPublication Number 2007-0276445 and titled SYSTEMS AND METHODS FORSENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE; copendingU.S. patent application Ser. No. 10/901,258, filed Jul. 27, 2004 andentitled MULTIPLE ELECTRODE VECTORS FOR IMPLANTABLE CARDIAC TREATMENTDEVICES, now U.S. Pat. No. 7,392,085; and U.S. Pat. No. 6,988,003entitled OPTIONAL USE OF A LEAD FOR A UNITARY SUBCUTANEOUS IMPLANTABLECARDIOVERTER-DEFIBRILLATOR, the disclosures of which are incorporatedherein by reference. The inclusion of each of functions 106 may varydepending upon the particular device.

After device initialization at 104, the device is tested as indicated at108. For an implantable cardioverter defibrillator (ICD), for example,fibrillation may be induced in the patient in order to determine whetherthe ICD accurately senses the fibrillation and successfully deliverstherapy that returns the patient to normal cardiac rhythm. If testing isunsuccessful, or if the device does not initialize, the system may beexplanted. If, instead, the initialization at 104 is completedsuccessfully and the system passes device testing at 108, then thesurgical portion of the implantation is completed, as shown at 110, withappropriate methods including closing incisions made duringimplantation, etc.

In the illustrative method, postural analysis 112 follows completion ofthe surgical portion of the implantation method. During posturalanalysis 112, after the patient has had the device inserted andactivated, and, possibly, after the patient has had time to recuperatesomewhat, the operation of the implanted device is observed and may bemodified. In particular, the patient may be asked to assume a series ofpositions, for example, sitting down and then standing up, while theimplanted device gathers data to determine which of its availablesensing vectors is best suited to permanent operation. In oneillustrative example, the patient is led through a series of bodypositions such that the device may determine a single sensing vector foruse as a primary or default sensing vector all the time, such thatchanges in sensing operation do not have to occur whenever the patientchanges posture or body position.

In another illustrative example, an optimal or best vector is determinedfor each of the several body positions, and body position is monitoredduring operation such that the implanted device may select the optimalvector for a patient's current body position. Body position may bemonitored, for example, by the provision of physical sensors that detectbody position by reference to gravity, to movements, or the like.Transthoracic impedance can be used to provide a measure of patient bodyposition, for example. An activity sensor may be used to infer whetherthe patient is standing versus lying down. Alternatively, body positionmay be monitored by observation of captured electric signals. Forexample, during postural analysis 112, the system may be configured toidentify signal markers to differentiate cardiac signals captured whilethe patient is in each of several body positions and, thereafter, toidentify the patient's body position by observation of the patient'scardiac signals.

Once postural analysis 112 is complete, the patient may then bedischarged 114, as the implantation procedure and process is thencomplete. Following discharge 114, the patient may be requested toreturn for further diagnostics, for example, initialization may beupdated (such as functions 106) and the postural analysis 112 may belater repeated. This may be done, for example, as the patient'sphysiology changes due to reaction to the implantation itself, withchanges in patient medication, and/or as the patient ages.

FIGS. 3-5 show illustrative methods of postural assessment in animplanted medical device. FIG. 3 illustrates a confirmation approach topostural assessment, in which a vector is identified on the basis of itscharacteristics while the patient is in a given body position, and theidentified vector is then checked to confirm that it is usableregardless of the body position of the patient. The method begins atblock 130, with the patient in a given body position, in this instance,supine. In other embodiments, the patient may begin in a different bodyposition, such as prone, reclined, standing, or in whatever position thepatient prefers to sleep in, for example.

With the patient in the given position, a vector is selected, as shownat 132. A vector may be selected at 132, for example, on the basis ofthe signal-to-noise ratio (SNR) of cardiac signals captured along thatvector. Other metrics for selecting a vector at 132 may include signalamplitude, noise amplitude, etc. In an illustrative embodiment, acombination of SNR and signal amplitude are taken into consideration.For example, a formula using both SNR and amplitude may be used. Someillustrative examples of such analysis are shown in copending U.S.patent application Ser. No. 11/441,522, filed May 26, 2006, published asUS Patent Application Publication Number 2007-0276445 and titled SYSTEMSAND METHODS FOR SENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICALDEVICE.

In another illustrative embodiment, a default vector is assumed, ratherthan selected on the basis of any metric. For example, if a particularsystem and implantation has a vector that works for most patients, thenthat vector may be selected first to start analysis. In another example,there may be a preferred vector, such as where there are twosensing-only electrodes and one or more sensing/shocking electrodes. Dueto possible physical changes at the electrode/tissue interface for thesensing/shocking electrodes when stimulus is delivered, the vectorbetween the two sensing-only electrodes may be “preferred”. Thus, instarting the method, the “preferred” vector may be selected at step 132.

With the vector selected at 132, a template for analysis of cardiacsignals may optionally be formed, as shown at 134. This optional step ofthe method of FIG. 3 may include capturing a set of signals with theselected vector, identifying likely cardiac events in the set ofsignals, and forming windows of captured signals around fiducial pointslikely to correspond to cardiac events.

An illustrative example of template formation may be as follows. First,the signal captured along the selected vector may be compared to athreshold and, when the threshold is exceeded, a cardiac event isassumed to be likely. A peak value in a set of samples following thethreshold crossing (for example, the next 40 sequential samples,captured at 256 Hz) is identified as a fiducial point, and a window ofsamples around the fiducial point is calculated. This initial capture ispresumed to be a cardiac event. Several additional “events” aresubsequently captured in association with later threshold crossings,with the fiducial point and/or window being selected in a mannercorresponding to that of the original identified “event”.

The captured events are then compared to one another, for example, usingcorrelation waveform analysis, or alternatively, another metric such asQRS width, which may be identified as a duration in which the signalremains above a minimum threshold. If the set of events is sufficientlysimilar, then a template is formed by selecting a representative eventor by averaging a set of events. In some embodiments, the template maybe dynamically updated by averaging in later-captured events. Additionalexamples of template formation may also be found in copending U.S.patent application Ser. No. 10/999,853, filed Nov. 29, 2004 and entitledMETHOD FOR DEFINING SIGNAL TEMPLATES IN IMPLANTABLE CARDIAC DEVICES, nowU.S. Pat. No. 7,376,458. In some embodiments, template formation mayfail where similarity among sets of captured events does not occur. Forexample, a time-out may be used, and, if after a period of time (60-180seconds, for example), a template cannot be formed using similarityanalysis, the vector under consideration may be identified as poorlysuited to detection.

Next, the patient is directed to change body positions, as indicated at136. One or more additional positions beyond the original position maybe used. For example, if the patient is supine, as indicated at 130, thepatient may be asked to move to a prone position, to sit up, and/or tostand. The positions listed at 138 are merely illustrative, and no setnumber of such positions is necessary. In an illustrative embodiment, itis determined what positions the patient tends to rest in, for example,reclined, and/or the patient's sleep position, such as on the patient'sside, in order that the posture assessment considers those positions thepatient is in the most. For each position the patient is asked toassume, the implanted device analyzes the selected vector to determinewhether the selected vector will function adequately for cardiac eventdetection and analysis. In an illustrative example, if the selectedvector fails to function adequately in any position, the method skipsany remaining body positions and advances to step 140. Otherwise, theselected vector is analyzed at each of at least two positions.

In an illustrative example, the selected vector is analyzed in each ofthe positions by considering one or more of SNR, signal amplitude,and/or noise amplitude. For example, the SNR and signal amplitude may beconsidered by the use of a formula, as further explained below.

The method then goes to step 140, where it is determined whether theselected vector has been verified or confirmed as a “good” sensingvector. If so, the method ends at 142. If the selected vector is notverified or confirmed, the method returns to step 132 and selects adifferent vector, with the patient being asked to again assume the firstbody position upon return to step 132 and, thereafter, to move throughadditional body positions as directed at step 136. In this method, avector is first selected and then tested as the patient changes bodypositions.

FIG. 4 illustrates a deterministic approach, as contrasted to theconfirmation approach of FIG. 3. In FIG. 4, for example, the patientassumes a given body position and holds that position until data isgathered for each of the available vectors. The method of FIG. 4 beginswith start block 160. The patient is asked to select a body position, asindicated at 162. Illustrative positions are listed at 164; in anexample, the patient is asked to lay supine and is later asked to sitand/or stand. Each vector is then analyzed while the patient remains inthe selected body position, as shown at 166. After data is captured foreach vector during step 166, it is determined if all positions have beentested, as indicated at 168. If not, the method returns to step 162,where a different body position is selected and the patient is asked toassume a different body position. Once each desired body position istested, the method exits the loop from 168, and a vector is selected asindicated at 170. Once a vector is identified on the basis of the datacaptured in the loop of 162-166-168, the method may perform templateformation as indicated at 172. In some embodiments, if templateformation 172 fails, the method may return to step 170 and select adifferent vector. The method then ends, as indicated at 174.

In an alternative embodiment, first and second vectors are identified,with the second vector being a back-up vector for use either because thefirst, primary or default vector becomes unavailable or, alternatively,for use if the first vector provides ambiguous indications of whetherthe patient is experiencing an arrhythmia.

In some embodiments, a single vector is identified for use while thepatient is in any body position. In other embodiments, several templatesmay be formed for the patient. For example, analysis of data capturedwhile the patient is in a supine position may indicate a different“best” vector than that captured while the patient is an uprightposition. Two templates could then be formed, without regard for whetherthe first and second templates are formed using the same vector, whereeach template includes information for its sensing vector configuration.

Illustratively, the first template could use sensing vector A-Can (seeFIG. 1A) and could be defined at the time of postural assessment whilethe patient is supine, and the second template could use vector A-B (seeFIG. 1A) and could be defined at the time of postural assessment whilethe patient is sitting upright. Then, during analysis, the primarytemplate at any given time could be whichever indicates ordinary cardiacfunction. If neither template indicates ordinary cardiac function, itmay be presumed that an arrhythmia is occurring, and a stimulus may bedelivered. This may be performed as part of a Boolean approach tofinding arrhythmic activity:

IF Analysis(A) fails

AND IF Analysis(B) fails

THEN Event indicates Malignant Rhythm

This tiered analysis may prevent misdiagnosis of the cardiac rhythm dueto a change of the patient's body position.

In another illustrative example, the primary and secondary templates maybe “switched” one for the other in response to an output from a bodyposition sensor. For example, if the primary template is associated witha first body position and the secondary template is associated with asecond body position, an output of a body position sensor may bemonitored, and the templates switched depending upon the body positionindicated by the body position sensor.

Additionally, switching between the templates may be achieved withoutneeding the use of a dedicated position sensor. In an illustrativeexample, only the Analysis(A) is performed in a morphology-basedanalysis system until a first threshold of abnormality is met. Forexample, if an X out of Y counter is used, the Analysis(A) may be usedby itself until a first X out of Y threshold is met. In an illustrativeexample, if 18/24 events is the threshold for a determination that amalignant cardiac rhythm is occurring, then an 8/24 threshold may beused to activate Analysis(B). If 2-3 events occur before Analysis(B)begins functioning, then a smooth transition may occur if the patienthas changed positions since, by the time the 18/24 counter fills forAnalysis(A), 8 or more events will have been detected using Analysis(B).In an illustrative example, if 4/8 of the events using Analysis(B) arefound to meet template comparison parameters, the system may switch toAnalysis(B), using a different template and/or vector than Analysis(A)for primary analysis. In some illustrative embodiments that use bothprimary and secondary templates, stimulus delivery (or preparation forstimulus delivery) may be delayed until it is determined that theanalysis with each template indicates a malignant cardiac rhythm.

The method of FIG. 4 calls for more data to be gathered than the methodof FIG. 3, regardless of whether one vector is superior or not.Therefore, this method may be slower in some instances than that of FIG.3. However, with the method of FIG. 3, the patient may be asked toperform and repeat several movements during postural assessment. If adevice is implanted in a patient who is relatively weak, for example,due to advanced congestive heart failure, repeated movements may beundesirable. Illustrative examples may be configured to perform eithermethod. In another illustrative example, the devices (the implantedmedical device and/or the programmer) in the system are equipped toperform either method, with the programmer allowing a physician toselect one method or the other.

FIG. 5 illustrates a detailed method for an illustrative embodiment. Asindicated at 200, the method begins with the patient (PT) supine. Step200 may be a directive given from the programmer to a physician to havethe patient assume a supine position, or it may be given directly to thepatient from the programmer itself. With this position verified (forexample, the programmer may request an input indicating that the patientis in the requested position), a first vector is selected, as indicatedat 202.

A VS SCORE_(SUP) is then calculated, as indicated at 204. The VSSCORE_(SUP) may be a “score” calculated to indicate the quality of thesensing vector. As discussed above, this may include consideration ofsignal amplitude, SNR, etc. Calculation of a SCORE may make use of aformula, a look-up table, or any suitable method for placing a metric onthe quality of a sensing vector. Illustrative methods are shown incopending U.S. patent application Ser. No. 11/441,522, filed May 26,2006, published as US Patent Application Publication Number 2007-0276445and entitled SYSTEMS AND METHODS FOR SENSING VECTOR SELECTION IN ANIMPLANTABLE MEDICAL DEVICE.

After the score is calculated at 204, it is determined whether allvectors have been analyzed, as indicated at 206. If not, a differentvector is selected, as shown at 208, and the method calculates anotherVS SCORE_(SUP) at 204. If all the vectors have been considered at step206, the method continues to block 210.

At block 210, the programmer or the implantable device, depending on thesystem configuration, determines whether additional patient positionsare possible. If not, the method continues to step 212, where the vectorhaving the largest SCORE from the initial analysis is selected for usein analysis, and template formation is initiated, as indicated at 214.This opt-out step 210 may be provided to accommodate a patient who isnot capable of changing body positions due to physical limitations or toaccommodate use during implant procedures.

If, at step 210, one or more additional patient positions are possible,the method continues to step 220. At step 220, the programmer requeststhat the patient adopt a different position, for example, standing orsitting. The method then performs similar steps to those performed withthe first body position. A vector is selected at 222, a score, VSSCORE_(STD), is calculated at 224, and it is determined whether eachvector has been considered at 226. If not, as indicated at 228, themethod returns to 224 with a different vector selected. Once all vectorsare completed, the vectors are compared, as indicated at 230. In anillustrative embodiment, the formula shown at 236 is used to calculate aPASS_(Value) for each vector.

Formula 236 includes two major terms. FIGS. 6A-6B are graphs of variablerelationships in an illustrative method of analyzing sensing vectorsduring postural assessment, with FIG. 6A illustrating the first term:

√{square root over ((SCORE_(SUP))²+(SCORE_(STD))²)}{square root over((SCORE_(SUP))²+(SCORE_(STD))²)}−|SCORE_(SUP)−SCORE_(STD)|

The result of this first term increases with the amplitude of eachSCORE, and is greatest when the amplitudes of the SCOREs are similar.

FIG. 6B illustrates relationships for the second term:

$1 - \left( \frac{{{Max}\left( {{AMP}_{SUP},{AMP}_{STD}} \right)} - {{Min}\left( {{AMP}_{SUP},{AMP}_{STD}} \right)}}{{Max}\left( {{AMP}_{SUP},{AMP}_{STD}} \right)} \right)$

When the signal amplitudes are closest to one another, the value of thisterm approaches 1, while it approaches zero if the maximum signalamplitude of the sensing vector when the patient is in one body positionis significantly different from the maximum signal amplitude of thesensing vector when the patient is in the other body position.

In short, illustrative formula 236 takes into account the size andsimilarities of the SCOREs, i.e., whether the vector quality is high andthe strength of the signal along a given vector is high in the firstterm, and whether the vector provides relatively consistent output,particularly in terms of signal amplitude, without regard for the bodyposition of the patient. One reason for the inclusion of the second termis that, in the illustrative embodiment, the event detection system ofthe implantable device includes an amplifying input having two dynamicranges, one which is larger and one which is smaller. With such asystem, it may be better, in some embodiments, to select a vector thatcaptures signal that makes relatively full use of one of the dynamicranges in each of the patient body positions.

For example, given dynamic ranges of 0-2.0 mV and 0-4.0 mV, analysis maybe easier and more reliable with a first vector in which peak voltagesare 0.75 mV for both body positions, rather than a second vector inwhich peak voltages are 0.75 mV for one body position and 2.5 mV for asecond body position, as the latter vector would call for loweramplification to accommodate the second body position but would use thedynamic range poorly when the patient is in the first body position. Forexample, FIG. 7C illustrates a SCORE calculation in which the amplitudeaffects the output SCORE in a manner having first and second peaks, witha trough therebetween. While having both terms account for dynamic rangeis part of some illustrative embodiments, other illustrative embodimentsmay instead use the formula of FIG. 7A for score calculation, as furtherexplained below.

The illustrative formula 236 presumes a SCORE value is generated in somemanner. Some illustrative embodiments use the following approach:

SCORE=S _(A) ×S _(R)

Where, for example, S_(A) and S_(R) may be calculated using one ofseveral approaches. In one such embodiment, the following formula isused:

$S_{A} = {{GAIN}*\frac{\exp \left( {N\; 1*\left\lbrack {\ln \left( {{N\; 2*{QRS}_{Avg}} - {N\; 3}} \right)} \right\rbrack^{2}} \right)}{{D\; 1*{QRS}_{Avg}} - {D\; 2}}}$

Where:

GAIN = 64.0 N1 = −34.7222 N2 = 0.2326 N3 = −0.6047 D1 = 0.3008 D2 =−0.7821It should be noted that the limits for QRS_(Avg), in this illustrativeembodiment, are 0<QRS_(Avg)<4.0. A graph illustrating the relationshipbetween S_(A) and QRS_(Avg) is shown in FIG. 7A.

The illustrative Scoring method further includes calculating a valueS_(R) as:

S _(R) =C _(R)*(SNR)²

Where: if SNR≦5 3.5, C_(R)=0.1;

-   -   if 3.5≦SNR≦10, C_(R)=1; and    -   if SNR>10, S_(R)=100        A graph illustrating the relationship between S_(R) and the SNR        is shown in FIG. 7B.

In another embodiment, S_(A) and S_(R) may be calculated from thefollowing look-up table:

LOOKUP TABLE S_(A) QRS_(Avg) (mV) S_(R) SNR 0.5 ≦0.5 0.5 ≦3  5  0.5-0.651 3-3.5 10 0.65-0.8  25 3.5-4   18 0.8-1.0 50 4-5   30 1.0-1.7 75 5-7.520 1.7-2.0 100 >7.5 40 2.0-3.0 15 3.0-3.5 0.5 3.5-4.0It can be seen that the output scores using the lookup table wouldinclude a trough in the range of 1.7 mV<QRS_(Avg)<2.0 mV, in accordancewith an embodiment adapted for multiple dynamic ranges.

In yet another embodiment, S_(A) and S_(R) are calculated using anotherpolynomial formula, for example:

S _(R) =C _(R)*(SNR)²

where: if SNR≦3.5, C_(R)=0.1;

-   -   if 3.5<SNR≦10, C_(R)=1; and    -   if SNR>10, S_(R)=100        It can be seen that S_(R) is calculated the same here as in the        first calculation method shown above, and a graph showing the        relationship is shown in FIG. 7B. In this illustrative example,        S_(A) may be calculated using the following:

S _(A) ={C ₁*(QRS _(Avg))⁶ +C ₂*(QRS _(Avg))⁵ +C ₃*(QRS _(Avg))⁴ +C₄*(QRS _(Avg))³ +C ₅*(QRS _(Avg))² +C ₆*(QRS _(Avg))+C ₇}

where, if QRS_(Avg)≦2.0,

C₁ = 22.5718 C₂ = −105.9666 C₃ = 160.2345 C₄ = −88.9262 C₅ = 29.6019 C₆= −1.2859 C₇ = 0.0087and, if QRS _(Avg)>2.0,

C₁ = 56.5544 C₂ = −1069.9959 C₃ = 8310.0056 C₄ = −33849.9682 C₅ =76139.7271 C₆ = −89551.3405 C₇ = 43035.7880A graph illustrating the relationship between S_(A) and QRS_(AVG) isshown in FIG. 7C. Both the lookup table and this third method using a6^(th) order polynomial take into account a system having first andsecond dynamic ranges by providing a dip in the SCOREs corresponding toinput signals that would border between the two dynamic ranges.

These methods of calculating SCOREs are merely illustrative, and thoseof skill in the art will understand that the values and calculationsinvolved will vary depending upon the positioning of the system, theelectronics and electrodes used, power level(s), and, potentially, othervariables.

It is sufficient for the present invention that an analysis of a sensingvector is performed with the patient in two or more body positions, iftwo positions are possible, and that this analysis provides a resultthat indicates whether or not the vector is useful/usable. For someembodiments, the analysis may further provide results for severalvectors such that the vectors may be compared to one another. Forexample, in the method of FIG. 3, a Boolean output of Yes/No may be aresult of vector analysis such that, if a vector is initially selectedand is functional, a Yes output results. For the method embodiments ofFIGS. 4-5, however, an output of vector analysis that allows comparativeanalysis is illustrated, as it allows the vectors to be compared to oneanother at the end of analysis. In alternative embodiments, a method asin FIGS. 4-5 returns Boolean results, with the available vectorsprioritized such that the highest priority vector returning a functionalresult (Yes output, for example) is selected.

Referring again to FIG. 5, after comparison of the vectors at 230, thevector with the largest PASS_(Value) is selected as the default orprimary sensing vector, as indicated at 232. Template formation may thenbe initiated, as indicated at 214, and the device may move on toordinary function. If desired, a secondary sensing vector may also beidentified in step 232. A secondary sensing vector may be used, forexample, for any of the reasons set forth above, including to resolveambiguities, to provide a Boolean check of cardiac function, or toprovide a second vector to use when the patient changes body position.

In a more general embodiment making use of similar relationships, thefirst formula could be of the form:

$\sqrt{\sum\limits_{i = 1}^{i = n}\left( {SCORE}_{n} \right)^{2}} - {\quad\left\lbrack {{{Max}\left( {{SCORE}_{1}\ldots \; {SCORE}_{n}} \right)} - {{Min}\left( {{SCORE}_{1}\ldots \; {SCORE}_{n}} \right)}} \right\rbrack}$

where n is the number of body positions tested, and SCORE_(i) is thescore for the vector while the patient is in the i^(th) body position.The amplitude factor may be analogous to the above amplitude factor. Anadditional factor which may be included takes the form:

$\frac{{Min}\left( {{SCORE}_{1}\ldots \; {SCORE}_{n}} \right)}{{Max}\left( {{SCORE}_{1}\ldots \; {SCORE}_{n}} \right)}$

This term would further emphasize a minimum score value, for example,when using three body positions in the postural assessment, if a verylow score is achieved in any of the positions, the vector underconsideration may be poorly suited to detection regardless of highSCOREs in the other positions, and this factor would reduce the outputPASS value.

FIGS. 8A-8E are graphical representations of display outputs of aprogrammer during an illustrative method of postural assessment. In theillustrative embodiment, the programmer includes a touch screen; inother embodiments, the programmer may include buttons, a keypad or othercontrols, and may take any suitable form.

FIG. 8A shows a first screen shot. The programmer indicates to thephysician that the postural assessment procedure is to start. Thephysician is asked to ensure that the patient is laying down (a firstbody position) and then to touch the “continue” icon 300 on the screen.Across the top of the programmer screen, status is indicated for theimplanted device. In particular, during these steps, therapy for thedevice may (optionally) be turned off. The device status as “implanted”is indicated, as is the patient's heart rate. In some embodiments, theprogrammer may refuse to perform postural assessment if the patient'sheart rate is not in a predetermined range, for example, between 50 and120 bpm.

After the physician presses the “continue” icon 300 from the screen shotof FIG. 8A, the next screen that is seen is that of FIG. 8B, in whichthe programmer indicates that the device is collecting the patient'srhythm. The physician is requested to keep the patient still. A cancelicon 302 is provided in case the physician determines that the procedureshould stop, for example, if the patient feels uncomfortable or ill,displays a physical abnormality (rising heart rate, for example), or ifthe patient moves. If desired, a status bar may be provided to indicatethe progress of the sensing vector analysis to the physician.

Once data is captured for the first patient body position, the nextscreen shot is that of FIG. 8C. The physician is asked to have thepatient sit up (a second body position). With the patient sitting up,the physician is asked to keep the patient still and depress thecontinue icon 304. The programmer then displays the screen shot of FIG.8D, which is quite similar to that of FIG. 8B and again includes anoptional cancel button 306 and may include a status bar.

As shown in FIG. 8E, once data capture is complete for the second bodyposition, patient tailoring in the illustrative embodiment is complete.The physician (or other operator) may go on to perform other tasks bytouching the “continue” icon 310. In other embodiments, additional datacapture may ensue, if desired.

In some embodiments, the data capture for the patient may includeoptions for physician input. For example, during data capture, it ispossible for an artifact (such as a T-wave) to interfere with detectionof R-waves. In some instances, the physician's input may be needed orrequested to resolve any questions relating to event classification.Some examples are discussed in copending U.S. patent application Ser.No. 11/441,522, filed May 26, 2006 published as US Patent ApplicationPublication Number 2007-0276445 and entitled SYSTEMS AND METHODS FORSENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE. In oneillustrative embodiment, any such questions are delayed until all datahas been captured, allowing the physician to concentrate on theprogrammer to answer such questions, rather than having the patientremain in a predetermined position while such issues are resolved. Inanother illustrative embodiment, such questions are asked as they arise.

In some embodiments, the above methods may be revisited later, afterimplantation, by the patient on his or her own. For example, ashome-monitoring systems become available for patients with implantedcardiac monitoring and/or stimulus devices, a home monitoring system maybe used to communicate with the implanted device, allowing laterre-selection of sensing vectors in light of postural assessment. Thehome monitoring system, in an illustrative example, may include agraphical user interface allowing the user to indicate readiness forpostural assessment, after which the home monitoring system may providegraphical output indicating what to do, physically, for the patient tocomplete home-monitoring self-assessment of postural effects on cardiacsignals. Thus, a home monitoring system having functionality allowing itto provide patient directions in support of postural assessment may alsobe a “programmer” in the methods and systems discussed herein.

Yet another illustrative embodiment may include a device as shown ineither of FIGS. 1A-1B which includes a sensor or sensing system fordetermining patient body position. For example, the sensor may be agravity sensor or accelerometer-type sensor. The sensor may also be usedto measure transthoracic impedance as a surrogate for patient bodyposition. In one embodiment, the system need not determine an actualposition using the position sensor, but instead may identify positionsensor output ranges that correlate to the usefulness of particularvectors and/or templates. For example, when a patient moves from sittingto standing, a position sensor output may indicate a change of position.When the position changes as indicated by the position sensor, thesystem may determine for itself whether the primary sensing vectorshould be changed by observing various template analyses. To this end,from the implanted medical device system perspective, knowledge of theposition is not needed, but instead, identification of the best templatefrom several that are available is sufficient.

During operation, another illustrative implanted medical device mayperiodically (at intervals) or occasionally (in response to a conditionor request) perform postural assessment without requesting movement bythe patient. For example, if the position sensor output is “X”, vectorselection may be performed. If the position sensor later provides adifferent output, “Y”, vector selection may again be performed, as ismay be presumed that the patient is in a different body position. Thisprocess may be repeated several times, with templates and vectorsidentified for various position sensor outputs. After the selectionprocess, if the position sensor output returns to “X”, then a vectorand/or template associated with position sensor output “X” may beselected. Within this approach, a single vector may have multipletemplates, each corresponding to a position sensor output.

Those skilled in the art will recognize that the present invention maybe manifested in a variety of forms other than the specific embodimentsdescribed and contemplated herein. Accordingly, departures in form anddetail may be made without departing from the scope and spirit of thepresent invention as described in the appended claims.

1. An implantable cardiac stimulus system comprising: an implantable medical device including a canister housing operational circuitry and at least three electrodes coupled to the operational circuitry for sensing cardiac activity while implanted; and a programmer configured to communicate with the implantable medical device after the implantable medical device is implanted in a patient; wherein the system is configured to perform the following method after the implantable medical device is implanted in a patient such that the at least three electrodes define at least first and second sensing vectors analyzable by the operational circuitry: the programmer indicates that the patient is to hold a first body position while a first analysis is performed; the implantable medical device captures data using at least the first and second sensing vectors; the implantable medical device indicates to the programmer that a first data capture for sensing vector analysis using the first and second sensing vectors is complete; the programmer, once the implantable medical device indicates completion of the first data capture for sensing vector analysis, indicates that the patient is to hold a second body position while a second analysis is performed; the implantable medical device again captures data using at least the first and second sensing vectors; the implantable medical device indicates to the programmer that a second data capture for sensing vector analysis using the first and second sensing vectors is completed; and the programmer indicates that the second analysis is complete.
 2. The system of claim 1 wherein the implantable medical device is configured to perform analysis of the data captured during at least the first and second data captures and to select a primary sensing vector for use in cardiac event detection regardless of the patient's body position.
 3. The system of claim 1 wherein the implantable medical device is configured to transmit data to the programmer from the first and second data captures, and the programmer is configured to select one of the sensing vectors as a primary sensing vector for use by the implantable medical device to perform cardiac event detection regardless of the patient's body position.
 4. The system of claim 1 wherein the implantable medical device is configured to perform analysis directed toward identifying cardiac events in a sensed data stream and, if difficulties occur in identifying cardiac events in the captured data, the implantable medical device is adapted to identify such difficulties, and communicate data to the programmer related to such difficulties and, if data related to difficulties in identifying cardiac events in the captured data is communicated to the programmer, the programmer is adapted to present one or more queries to an operator of the programmer in order to resolve such difficulties.
 5. The system of claim 1 wherein the implantable medical device identifies primary and secondary sensing vectors.
 6. An implantable cardiac stimulus system comprising: an implantable medical device including a canister housing operational circuitry and at least three electrodes coupled to the operational circuitry for sensing cardiac activity while implanted; a programmer configured to communicate with the implantable medical device when the implantable medical device is implanted in a patient; wherein the programmer is configured to request that: the patient hold a first body position while the implantable medical device performs first analysis from an implanted configuration defining at least two sensing vectors between the at least three electrodes, and the patient hold a second body position while the implantable medical device performs second analysis from the implanted configuration; and the implantable medical device is configured to: capture data corresponding to the first and second analysis and indicate completion of the first and second analysis to the programmer such that the programmer may end corresponding requests; select a first template representative of detection along a first template sensing vector of non-malignant cardiac activity while the patient is in the first body position; select a second template representative of detection along a second template sensing vector of non-malignant cardiac activity while the patient is in the second body position; and use the first and second templates to determine whether a malignant cardiac rhythm is occurring.
 7. The system of claim 6, wherein the implantable medical device is configured to use the first and second templates to determine whether a malignant cardiac rhythm is occurring by the following: comparing a captured event to the first template and, if the comparison indicates substantial dissimilarity therebetween: comparing the captured event to the second template and, if the comparison indicates substantial dissimilarity therebetween: determining that the captured event indicates a malignant cardiac rhythm; and determining whether a substantial number of captured events in a defined set of captured events indicates a malignant cardiac rhythm.
 8. An implantable cardiac stimulus system comprising: an implantable medical device including a canister housing operational circuitry coupled to at least three electrodes; a programmer configured to communicate with the implantable medical device when the implantable medical device is implanted in a patient; wherein, once the implantable medical device is implanted in a patient such that the at least three electrodes define at least first and second sensing vectors analyzable by the operational circuitry, the system is configured to select a first sensing vector for analysis and perform the following steps to analyze the first sensing vector: a. the programmer indicates that the patient is to hold a first body position while a first analysis is performed; b. the implantable medical device captures data using the selected sensing vector; c. the implantable medical device indicates to the programmer that a first data capture for sensing vector analysis using the selected sensing vector is completed; d. the programmer, once the implantable medical device indicates completion of the first data capture, indicates that the patient is to hold a second body position while a second analysis is performed; e. the implantable medical device captures data using the selected sensing vector; f. the implantable medical device indicates to the programmer that a second data capture for sensing vector analysis using the selected sensing vector is completed; and g. the programmer indicates that the second analysis is complete.
 9. The system of claim 8, further configured to determine whether the selected sensing vector is suitable for detection of cardiac events in each of the body positions and, if not, the system is also configured to select a second sensing vector for analysis and repeat steps a-g. 